1. Field of the Invention
The present invention relates to an image pickup element for picking up an object image.
2. Related Background Art
FIG. 1 shows a structure of a general image pickup element 61 for forming a color image, which has been used up to now. Reference numeral 501 denotes a silicon wafer; 502, a microlens; 503, a photoelectric conversion section having a function for converting a received photon into an electric charge; 504 (504r and 504g), color filters for separating wavelengths of a light; and 510, a Poly wiring layer serving as a gate for controlling an electric charge of the photoelectric conversion section. Reference numerals 511 to 513 denote wiring layers made of a metal such as aluminum. Among them, reference numeral 511 denotes an AL1 wiring layer serving as a connection between respective layers and an output line; 512, an AL2 wiring layer serving as a well power supply line and a control line; and 513, an AL3 wiring layer serving as a light shield and a power supply line. FIG. 2 is a view of the image pickup element 61 viewed from above.
Each pixel constituting the image pickup element 61 is provided with a primary color filter of R (red), G (green), or B (blue). In addition, as a color filter, there is a complementary color filter which uses three colors of C (cyan), M (magenta), and Y (yellow) as well. An image pickup technology is widely used in which: pixels are arranged in a mosaic shape; and then, luminance information and color information corresponding to the number of pixels are created by signal processing. As a color filter array of the image pickup element 61 used here, a structure called a Bayer array is often adopted. In FIG. 2, reference symbol 61nmg denotes a first green pixel; 61nmb, a blue pixel; 61nmr, a red pixel; and 61nmg2, a second green pixel. m indicates an array number of a pixel in a horizontal direction and n indicates an array number of a pixel in a vertical direction. These are regularly arranged as shown in FIG. 2 to constitute one image pickup element. In such a pixel array, the number of green pixels is twice as many as that of blue or red pixels. Although it is basically possible to create a color image if there are the same number of pixels for each of three colors, an image quality can be improved by increasing the number of green pixels having a rather high visibility.
However, the above-mentioned conventional example has problems as described below. In general, image pickup for obtaining satisfactory image characteristics consists of a first process for forming an object image with an optical apparatus, a second process for adjusting a high frequency component of a spatial frequency characteristic of the object image to suppress the component, a third process for photoelectrically converting the object image whose spatial frequency characteristic has been adjusted, and a fourth process for correcting a response according to a spatial frequency with respect to an obtained electric signal. In this case, since sampling of an optical image is performed with the image pickup element having a limited number of pixels, in order to obtain a high quality image output, it is necessary to reduce components of a Nyquist frequency or more peculiar to the image pickup element in the spatial frequency characteristic of the optical image in accordance with the sampling theorem. Here, the Nyquist frequency is a frequency that is a half of a sampling frequency depending upon a pixel pitch. Therefore, the optimized series of processes is for obtaining a high quality image, in which aliasing distortion is less conspicuous, that is, moiré is less conspicuous, by adjusting an optical image to be sampled such that it becomes an optical image-having a characteristic according to the Nyquist frequency peculiar to the image pickup element.
A modulation transfer function (MTF), which is a spatial frequency transmission characteristic of an image, is an evaluation amount with which a characteristic concerning a sharpness of a digital still camera, a video camera, or the like can be represented well. Specific factors affecting this MTF are an imaging optical system serving as an optical apparatus, an optical lowpass filter for band limitation of an object image, an opening shape of a photoelectric conversion area of an image pickup element, digital aperture correction, and the like. An overall MTF representing a final image characteristic is given as a product of MFTs of the respective factors. That is, it is sufficient to find MTFs of the above-mentioned first to fourth processes and calculate a product of the MTFs.
However, since digital filter processing as the fourth process is applied to an image output which has already been sampled by the image pickup element, it is unnecessary to take into account high frequencies exceeding the Nyquist frequency.
Therefore, the structure for reducing components of the Nyquist frequency or more peculiar to the image pickup element in the spatial frequency characteristic of the optical image means that components of the Nyquist frequency or more are small in the product of MTFs of the first, second, and third processes excluding the fourth process. Here, in the case in which image pickup is performed on the premise that a still image is viewed as in a digital still camera, it is necessary to take into account the fact that an image with a higher resolution is easily obtained when a response in a frequency slightly below the Nyquist frequency is higher even if high frequencies exceeding the Nyquist frequency are not eliminated but remain more or less.
In the formation of an object image with the imaging optical system, which is the first process, in general, an optical aberration is easier to be corrected in a center of a screen than in a peripheral part of the screen. In the center of the image in order to obtain a satisfactory image in the peripheral part of the screen, it is necessary to obtain an extremely satisfactory characteristic close to a diffraction limit MTF, which depends upon an F number of an imaging lens. In recent years, since miniaturization of pixels of an image pickup element has been advanced, this necessity is increasing. Therefore, it is better to consider the MTF for the imaging optical system assuming that it is an ideal aplanatic lens.
In addition, in an image pickup element in which light receiving openings with a width d are laid without a gap, since the width of the light receiving openings coincides with a pixel pitch, a response value of the third process at the Nyquist frequency u=d/2 is rather high. Therefore, it is a general practice to trap the vicinity of the Nyquist frequency in the second process in order to decrease a total MTF in the vicinity of the Nyquist frequency.
In the second process, an optical lowpass filter is usually used. A material having a birefringent characteristic such as a rock crystal is utilized for the optical lowpass filter. In addition, a phase-type diffraction grating as disclosed in Japanese Patent Application Laid-Open No. 2000-066141 may be utilized.
When a birefringent plate is placed in an optical path of an optical apparatus and is arranged to be slanted such that its optical axis is in parallel with a horizontal direction of an imaging surface, an object image according to ordinary light and an object image according to extraordinary light are formed deviating from each other in the horizontal direction by a predetermined amount. Trapping a specific spatial frequency with the birefringent plate means that a bright part and a dark part of a stripe of the spatial frequency are shifted so as to be overlapped with each other. An MTF according to the optical lowpass filter is represented by expression (1).R2(u)=|cos(π·u·ω)  (1)
Here, R2(u) represents a response, u represents a spatial frequency of an optical image, and ω represents an object image separation width.
If a thickness of the birefringent plate is selected appropriately, it is possible to eliminate the response in the Nyquist frequency of the image pickup element. In the case in which the diffraction grating is utilized, the same effect can be obtained through diffraction by separating the optical image into a plurality of images having a predetermined positional relationship and superimposing the images.
However, it is necessary to grow a rock crystal or a crystal of lithium niobate or the like and then grind it to be thin in order to manufacture the birefringent plate, which presents a problem in that the birefringent plate is extremely expensive. In addition, even if the diffraction grating is utilized, since a highly precise microstructure is required of the diffraction grating, it is still expensive.
Next, as to an efficiency of utilization of light, for example, in a CCD image pickup element in which pixels with a primary color filter, which is considered to have a high color reproducibility, are arranged in a mosaic shape, one each of optical filters of R (red), G (green), and B (blue) are arranged between a microlens 2 and a photoelectric conversion area 3.
In this case, in a pixel with the red optical filter, only red light are photoelectrically converted and blue light and green light are absorbed by the optical filter to be heat. In a pixel with the green optical filter, the blue light and the red light are not photoelectrically converted and become heat in the same manner. In a pixel with the blue optical filter, the green light and the red light are not photoelectrically converted and become heat in the same manner. That is, only light, which have been transmitted through a predetermined optical filter, in an incident light beam are photoelectrically converted and outputted as an electric signal in each pixel of the conventional color image pickup element. Thus, light which could not be transmitted through the optical filter are disposed as heat or the like.
FIG. 3 shows a spectral transmittance characteristic of R, G, and B color filters in an image pickup element. Since a transmittance of an infrared light is high, an infrared light cut filter for blocking a wavelength of 650 nm or more is actually further laid over these color filters between the image pickup element and an image pickup lens. As it is seen from this, only approximately one third of visible light are used effectively in one pixel.
Considering an efficiency of utilization for each color of R, G, and B more in detail, for example, an area ratio of R, G, and B pixels of a color image pickup element of the Bayer array shown in FIG. 13 is ¼: 2/4:¼ in case that an area of one unit constituting a regular array is assumed to be one. Thus, a utilization ratio of green light in case that a total amount of light is assumed to be one is ⅓× 2/4=⅙ as a product of a term of wavelength selectivity and a term of an area ratio, and that of red light and blue light is ⅓×¼= 1/12. In total, a utilization ratio of the light is ⅙+ 1/12+ 1/12=⅓. Therefore, the efficiency of utilization is still ⅓. On the contrary, when a total amount of light is assumed to be one, ⅔× 2/4=⅓ of the green light and ⅔×¼=⅙ of the red light and the blue light are not effectively utilized.
The above description is made referring to the image pickup element using the color filters of primary colors. In an image pickup element using complimentary color filters, approximately ⅓ of visible light are not photoelectrically converted and effectively utilized. In this way, no matter whether the color filters of primary colors or the color filters of complimentary colors are used, an efficiency of utilization of light is low in a conventional single plate image pickup element because an image pickup surface is divided by the color filters.
In addition, a structure for efficiently taking in light incident on an image pickup element is disclosed in Japanese Patent Application Laid-Open No. 06-224398 (FIG. 4). FIG. 5 shows a result of tracing incident light in the structure described in this patent application. Reference numeral 16 denotes a cap layer consisting of resin, which is formed of a material having a refractive index of approximately 1.6. Reference numeral 17 denotes a low refractive index layer, which is formed of resin having a refractive index lower than that of the cap layer 16 or formed in hollow (in which the air or an inert gas such as nitrogen is filled). Consequently, since an interface between the cap layer 16 and the low refractive index layer 17 becomes a surface on which light tends to be totally reflected, it is intended to take in oblique-incident light.
However, the above-mentioned conventional example has problems as described below. When a shape of an incidence section, which is the interface between the cap layer 16 formed of a high refractive index material and the low refractive index layer 17 and is on the object side, is an R shape as in FIG. 4, even if a part of oblique-incident light are reflected on the R surface, the light form an angle exceeding a total reflection condition on a surface on the opposite side and directly passes through the R surface to the low refractive index layer 17 as show in the result of tracing light of FIG. 5. The light having passed through the R surface may enter an adjacent pixel, which may also become a problem.